A number of technologies are used in speed sensor devices to determine the rotational speed of various components. For example, in various types of vehicles, speed sensor devices may be used with anti-lock braking systems (“ABS”). One specific speed sensor device is disclosed in application Ser. No. 10/874,838, the entire contents of which are incorporated herein by reference.
Conventionally, magnetic encoder wheels may be installed in motor vehicles. An encoder wheel typically includes an outer surface having magnetized ferrite contained therein. Specifically, the magnetized ferrite is contained in a laminate with alternating north and south poles, and the laminate extends around the circumference of the encoder wheel. The encoder wheel may include features that extend from the surface and project towards a magnetic sensor device. As the wheel rotates, the magnetic sensor may measure the change in the strength of the magnetic field of the alternating poles. From this alternating magnetic field and, in turn, output from the sensor device, the rotational speed of the encoder wheel can be calculated. Such calculations may use, for example, the correlation between the peaks of the sensor's output and the number of the alternating poles around the encoder wheel.
One type of magnetic sensor device for use with speed detection includes a Hall effect transducer. A Hall effect transducer uses a transverse current flow that occurs in the presence of a magnetic field. Applying a direct current (DC) voltage at both ends of the Hall effect transducer creates a longitudinal current flow over its body. With the presence of the magnetic field, a transverse voltage is induced in the Hall effect transducer that can be detected by a second pair of electrodes transverse to the first pair. The potential created in the sensor surface generates a sine wave output with respect to the rotation of the encoder wheel.
FIG. 1A is a schematic view of an example motor vehicle having an encoder for use with a speed sensor, and FIG. 1B shows an enlarged schematic view of a portion of an encoder wheel illustrating the magnetization on the encoder wheel. The encoder wheel 10 is coupled to an axle shaft 20 which rotationally drives a wheel 22. An anti-lock braking system 14 is located on the encoder wheel 10, and it includes a magnetic sensor 16 and a vehicle control module 18. The encoder wheel 10 is magnetized such that it includes a plurality of alternating north and south magnetic poles around its circumference. When the encoder wheel 10 rotates, the magnetic field changes. The magnetic sensor 16 will detect the change in the magnetic field and induce a voltage. The vehicle control module 18 will calculate a speed signal and transfer this information to the anti-lock braking system 14.
Unfortunately, such techniques suffer several drawbacks. For example, it is difficult and expensive to manufacture a system that includes alternating magnetic north and south poles disposed around the circumference of an encoder wheel. Such systems also are complex. Moreover, to control the vehicle, it is necessary to detect the vehicle's rotational speed or acceleration speed, while these techniques only detect the speed of the vehicle.
Thus, it will be appreciated that there is a need for an improved system that does not suffer from one or more of the above-mentioned drawbacks.